Now, a British study has gone a long way towards answering that question. When middle-aged Londoners were forced to walk in either green and lovely Hyde Park, or along traffic-clogged Oxford Street nearby, their hearts and lungs spoke the truth. Breathing in that pollution was bad.

“In all participants, irrespective of their disease status, walking in Hyde Park led to an increase in lung function,” the researchers wrote in the Lancet medical journal. And pulse wave velocity — a measure of stiffened arteries — fell in everyone. The benefits lasted a full day.

“By contrast, these beneficial responses were attenuated after walking on Oxford Street,” Rudy Sinharay of Imperial College, London and colleagues wrote.

It might seem obvious, but it hasn’t been clear if the drawbacks of air pollution counteract the benefits of exercising.

“Our findings suggest that healthy people, as well as those with chronic cardiorespiratory disorders, should minimize walking on streets with high levels of pollution because this curtails or even reverses the cardiorespiratory benefits of exercise,” the researchers wrote.

“Instead, walking exercise should be enjoyed in urban green space areas away from high density traffic.”

The researchers recruited 120 volunteers, 60 years and older, 80 of whom who had mild heart or lung disease.

“They’re the canaries in the coal mine,” said George Thurston, an expert on the health effects of air pollution at the New York University School of Medicine. The volunteers are a little more vulnerable to the effects of pollution than the average healthy person 20 years younger.

“Participants were randomly assigned by drawing numbered disks at random from a bag to do a two-hour walk either along a commercial street in London (Oxford Street) or in an urban park,” the researchers wrote in their report, published in the Lancet medical journal.

That was important, because other studies trying to tease out the effects of exercise, or pollution, or both, haven’t been able to show whether there is something different about people who choose not to exercise, or those who live in polluted areas.

“This very interesting new study gets around that limitation, by getting the participants to do things they wouldn’t necessarily have chosen to do as part of their normal activity. That is, it’s a real experiment,” said Kevin McConway, emeritus professor of applied statistics at The Open University in Britain, who was not involved in the study.

The team measured how much pollution the volunteers breathed in and measured heart and lung function. Even though London is not one of the world’s most notoriously polluted cities, there is a lot of diesel pollution in downtown streets such as Oxford Street, and there is much less in nearby Hyde Park, with its trees, bushes and grassy spaces.

“Hyde Park is green, has beautiful trees, and is rather tranquil for a central London location," McConway said. "Is it possible that the differences in response were caused by something other than the differences in air pollution and noise? Perhaps it’s simply more relaxing to walk in the park.”

But the researchers did measure exposure to pollutants. Typical pollutants that come from car and truck exhaust, including ultrafine particles and nitrogen dioxide, were clearly linked with reduction in lung function and less flexible arteries, they found.

“The benefits of exercise disappear at highly polluted locations.”

This finding did not surprise Thurston, who wrote a commentary on the findings in the Lancet.

“I am in Manhattan and a lot of times you see people running along very trafficked places,” he said

“The exercise is good for you but the higher the pollution levels, the less helpful it is. The benefits of exercise disappear at highly polluted locations.”

Thurston said people worried about whether pollution is counteracting the benefits of their exercise can find pollution monitors that link to smartphones. Or they can just work out away from traffic.

Not everyone has that luxury.

“The observation that air pollution encountered on a high street in London removes any health protection produced by exercise outdoors is yet another demonstration that pollution is eroding the health of ordinary people,” commented Stephen Holgate and air quality expert at the University of Southampton